Sort by Column (Descending)

Context. Show products in order of descending revenue for a top-line report.

Your task. Write sort_by_revenue_desc(df) that returns the DataFrame sorted by revenue in descending order.

Example. revenues [150, 50, 200, 100] → reordered as 200, 150, 100, 50.

Notes. Stable sort — equal revenues keep their original relative order. Empty df → empty df.